The U.S. patents of Gordon et al., U.S. Pat. No. 4,700,339, granted Oct. 13, 1987 and Mollenauer, U.S. Pat. No. 4,881,790, granted Nov. 21, 1989, are examples of the literature describing the advantages of using solitons for long distance optical fiber transmission of information. Solitons are pulses of light having a particular shape, width and energy that allows each pulse to propagate great distances on single-mode optical fiber while maintaining its shape and pulse width. For propagation on single-mode glass optical fibers, such pulses would ideally have a hyperbolic secant squared intensity envelope. Such a pulse causes certain nonlinear effects of the single-mode fiber to compensate for the effects of chromatic dispersion in the fiber. Pulses having the smallest possible time-bandwidth products for the spectra given, as is characteristic of ideal soliton pulses, are referred to as "transform limited pulses." Among other advantages of soliton transmission is the practicality of using erbium amplifiers, rather than repeaters, for amplifying the transmitted signal; soliton transmission also permits wavelength division multiplexing and polarization division multiplexing, which can further increase transmission line capacity.
The papers, "Mode-Locked Erbium-Doped Fiber Laser with Soliton, Pulse Shaping," J. D. Kafka and T. Baer, Optics Letters , Vol. 14, No. 22, Nov. 15, 1989, pp. 1269-1271, and "Picosecond Pulse Generation from Actively Mode-Locked Erbium-Doped Fibre Laser," A. Takada, H. Miyazawa, Electronics Letters, Vol. 26, No. 3, Feb. 1, 1990, pp. 216-217, are examples of the literature that describe the use of harmonically mode-locked fiber ring lasers for generating short pulses. The ring lasers described use an optical pump for exciting an erbium fiber amplifier located in a closed loop resonator path. An electrooptic modulator in the path is driven at an appropriate frequency to form the laser light into pulses having a repetition rate harmonically related to the frequency spacing between adjacent resonant modes of the closed loop resonator path. Pulses of light circulating around the closed loop of the ring laser are coupled out from the closed loop and constitute the pulses that can be used for soliton transmission. It can be shown that the pulses generated by actively mode locked lasers tend to have a Gaussian shape, but this pulse shape is sufficiently close to the desired hyperbolic secant squared pulses that they can be used in soliton transmission systems. Unfortunately, we have found that harmonically mode-locked ring lasers have a tendency to produce a train of pulses having amplitude variations. These pulse amplitude variations and other manifestations of instability are the result of inherent unwanted interaction between modes in the closed loop of the ring laser.
Accordingly, there is a continued need for practical optical pulse sources for use in optical fiber transmission, and particularly for practical soliton sources.